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Transcript of Chemistry
Cohesion and Adhesion among water molecules
plays a key role in the transport of water against gravity in plants
clinging of one substance to another, contributes too, as water adheres to the wall of the vessels.
sequoias
wood cut
Surface Tension
a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. Water has a greater surface tension than most
other liquids Water behaves as if
covered by an invisible film
Some animals can stand, walk, or run on water without breaking the surface
Polarity of Water
The molecule has opposite charges on opposite sides
polar molecules form H bonds with other polar molecules
Activity
Explain why water sticks to your skin when you take your finger out of the water.
Explain why these leaves float instead of submerging.
Name the property that makes water molecules attach to each other due to its surface charge.
Water is the Solvent of Life solution = solvent + solute(s)
ex: cup of coffee hot water (solvent) sugar (solute) coffee grounds (solute)
Aqueous solution The solvent is water
ramsey.k12.nj.us
Water Forms Hydrogen Bonds With Ions
The Na+ cations form hydrogen bonds with partial negative oxygen regions of water molecules.
The Cl- anions form hydrogen bonds with the partial positive hydrogen regions of water molecules.
Hydration shell - each dissolved ion is surroundedby a sphere of water molecules
Polar molecules are also soluble in water.
Even large molecules, like proteins, can dissolve in water if they have ionic and polar regions.
Hydrophilic
any substance that has an affinity for water Some are water-soluble
water molecules are attracted to molecules with an electrical charge (ionic or polar bonds)
H-bond formation may form
Hydrophobic
Substances that have no affinity for water Non-ionic & non-polar compounds are
repelled by water Water-insoluble Hydrophobic molecules are major
ingredients of cell membranes Some vegetables and fruits with non-
polar pesticides
Mole concept
Counting individual or even collections of molecules is not practical
Instead, we can use the concept of a mole to convert weight of a substance to the number of molecules in that substance and vice versa
Mole (mol)
Is equal in number to the molecular weight of a substance, but up scaled from daltons to grams Ex. a mole of sucrose (C12H22O11)
C = 12 daltons x 12 H = 1 dalton x 22 O = 16 daltons x 11 Sucrose molecular weight = 342 daltons one mole of sucrose = 342 g
Avogradro’s number
The actual number of molecules in a mole 6.023 x 1023
A mole of sucrose contains 6.023 x 1023 molecules weighs 342g
Molarity
the concentration of a material in solution one molar solution
has one mole of a substance dissolved in one liter of solvent, typically water.
Example: to make a 1 molar (1 M) solution of sucrose add water to 342 g of sucrose until the total volume is 1 liter and all the sugar is dissolved
2 H2O H3O+ + OH-
by convention: H2O H+ + OH- pH of pure water is 7 (10-7 M)
+
+-
hydronium hydroxide
Ionization of Water
pH
Scale 0-14 logarithmic scale
pH = - log10 [H+] acid pH< 7 base pH>7 neutral pH = 7
pure water at RT: pH = -log 10-7M
= -(-7) = 7
Neutral Solution
In a neutral solution [H+] = [OH-] = 10-7 M, and the pH = 7.
Values for pH decline as [H+] increase. While the pH scale is based on [H+],
values for [OH-] can be easily calculated from the product relationship
[H+] + [OH-] = 14
Acids
substances that increase [H+] in solution
pH below 7
ex: HCl H+ + Cl - acid precipitation, with pH < 5
acid indigestion use antacids
Bases
substances that reduce [H+] in solution
pH above 7 NH3 + H+ NH4 +
NaOH Na+ + OH-
ammonia ammonium ion
sodium hydroxide sodium hydroxyde ion ion
Activity
Rain has a pH of 5. Is it an acid, base or neutral? What is the concentration of H ions? What is the concentration of OH ions?